400-600 key-presses per minute
How do you do that? That's 10 per second
The keyboard buffer is hardware, it's located in the super I/O chip (I think it may be a part of the southbridge in present day mobos) and not very large. There's also a software buffer for the keyboard, the hardware one is maybe not really considered a buffer.
If you have done this on the same board with some other operating system without failure this means that Mint is not emptying the buffer fast enough.
I tried to figure out how to change this, but sometimes (more often than not these days it seems) I got to many irrelevant results when I searched for it, so....